So I'm doing an experiment to verify the contact duration when a steel rod (Material: Mild Steel) is allowed to free fall longitudinally on a steel plate. The arrangement is real simple:
The rod is allowed to fall longitudinally on the plate, resulting in a number of impacts. I will only be considering the first impact.
Now here's the problem I'm facing: During the first contact and off-contact, the oscilloscope reads a much higher than expected value for the contact time. Same procedure is repeated for three lengths of the rod (200mm, 350mm and 475mm), all resulting in the same anomaly. Here are the results:
As clear from the plot, the experimental values are similar to the theoretical values except for an offset (found to be about 121 µs). Any idea why this is happening?
P.S: The theory used is that of elastic waves produced on spherical contact where the impact results in the creation of a compressive wave that travels to the end of the rod and returns as a tensile wave which pulls the rod off contact. As such the contact time t= 2l/(wave speed). Wave speed = 5000 m/s.